Ceramic nanofiber separators

ABSTRACT

Provided herein are ceramic nanofibers and processes for preparing the same. In specific examples, provided herein are ceramic nanofiber mats for use as separators in batteries, particularly lithium ion batteries.

CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No.61/717,260, filed Oct. 23, 2012, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

Batteries comprise one or more electrochemical cell, such cellsgenerally comprising a cathode, an anode and an electrolyte. Lithium ionbatteries are high energy density batteries that are fairly commonlyused in consumer electronics and electric vehicles. In lithium ionbatteries, lithium ions generally move from the negative electrode tothe positive electrode during discharge and vice versa when charging. Inthe as-fabricated and discharged state, lithium ion batteries oftencomprise a lithium alloy (such as a lithium metal oxide) at the cathode(positive electrode) and another material, generally carbon, at theanode (negative electrode). Separators are generally porous, film-likematerials made of electrically insulating polymer olefins (such aspolypropylene (PP) or polyethylene (PE)).

SUMMARY OF THE INVENTION

Provided herein are ceramic-containing nanofibers, processes of usingsuch nanofibers, and processes of manufacturing such nanofibers. Inspecific embodiments, provided herein are separators comprisingnanofibers, such nanofibers comprising ceramic materials. For example,provided in certain embodiments herein is a nanofiber or a separatorcomprising a nanofiber (e.g., one or more nanofiber in the form of ananofiber mat), the nanofiber comprising a continuous ceramic matrix. Incertain embodiments, the ceramic matrix comprises at least two differentof materials, e.g., a first and second ceramic material. In someembodiments, the first and second ceramic materials form a first andsecond continuous matrix, or, collectively, form an integrated matrix.In some embodiments, the nanofiber further comprises a continuouspolymer matrix. In certain embodiments, the continuous ceramic matrix isan integrated matrix comprising ceramic and polymer.

In some instances, separators require porosity to allow flow of ionsbetween a cathode chamber and an anode chamber of a battery (e.g.,lithium cations in a lithium ion battery). However, it is also importantto consider the ability of the separators to minimize and preventrunaway reactions (e.g., resulting from ruptured membranes). Generally,ceramic materials have improved thermal stability compared to polyolefinmaterials (such as polyethylene and polypropylene) typically used inbattery separators, and nanofiber mats described herein generallyprovide sufficient porosity for lithium ions to pass through.

Provided in certain embodiments herein is a battery separator comprisinga nanofiber mat, the nanofiber mat comprising at least one nanofiber,the at least one nanofiber comprising at least one ceramic material, or,more specifically, at least two ceramic materials.

In specific embodiments, the nanofiber(s) comprises a continuous matrixof a ceramic material. In certain embodiments, the nanofiber(s) comprisecoaxially layered materials (e.g., two coaxially layered ceramicmaterials). In specific embodiments, the nanofiber(s) comprise a firstceramic material as a core material and a second ceramic material as asheath material, the sheath material at least partially surrounding thecore material. In various embodiments, each of the ceramic materials isindependently selected from the group consisting of silica, alumina,zirconia, beryllia, ceria, Mania, barium titanate, and strontiumtitanate.

In some embodiments, the nanofiber(s) provided herein is mesoporous. Incertain embodiments, the porosity of the nanofiber mat is at least 10%.In specific embodiments, the porosity of the nanofiber mat is at least20%. In more specific embodiments, the porosity of the nanofiber mat isat least 30%. In still more specific embodiments, the porosity of thenanofiber mat is at least 50%. In yet more specific embodiments, theporosity of the nanofiber mat is at least 80%.

In certain embodiments, a battery separator or nanofiber mat providedherein comprises non-aggregated, discrete domains of ceramic material.In specific embodiments, the nanofibers do not comprise a concentrationof domains 20 times higher along a 500 nm long segment along the lengthof the nanofiber than an adjacent 500 nm length of the nanofiber.

In some embodiments, the nanofiber(s) comprises at least 10% (e.g., atleast 30%) by weight of ceramic material (e.g., on average). In specificembodiments, the nanofiber(s) comprises at least 50% by weight ofceramic material (e.g., on average). In more specific embodiments, thenanofiber(s) comprises at least 70% by weight of ceramic material (e.g.,on average). In still more embodiments, the nanofiber(s) comprises atleast 90% by weight of ceramic material (e.g., on average). In yet moreembodiments, the nanofiber(s) comprises at least 90% by weight ofceramic material (e.g., on average). In certain embodiments, thenanofiber(s) comprises less than 50% by weight organic material (e.g.,on average). In specific embodiments, the nanofiber(s) comprises lessthan 30% by weight organic material (e.g., on average). In more specificembodiments, the nanofiber(s) comprises less than 10% by weight organicmaterial (e.g., on average). In still more specific embodiments, thenanofiber(s) comprises less than 5% by weight organic material (e.g., onaverage). In certain embodiments, the nanofibers comprise at least 50%by elemental weight metal and oxygen. In specific embodiments, thenanofibers comprise at least 60% by elemental weight metal and oxygen.In more specific embodiments, the nanofibers comprise at least 75% byelemental weight metal and oxygen. In still more specific embodiments,the nanofibers comprise at least 90% by elemental weight metal andoxygen.

In certain embodiments, the nanofiber(s) has an average diameter of lessthan 1 micron (e.g., less than 800 nm). In some embodiments, thenanofiber(s) has an average aspect ratio of at least 100 (e.g., at least1000 or at least 10,000).

Also, provided herein are batteries (e.g., lithium ion batteries)comprising any nanofiber described herein, or a separator comprising anyof the nanofibers described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 illustrates a multiaxial electrospinning (multiple fluids about asubstantially common axis) system for preparing a coaxially layerednanocomposite nanofiber, and a coaxially layered nanocompositenanofiber.

FIG. 2 illustrates SEM images of ceramic-containing nanofibers obtainedby (i) electrospinning a fluid stock prepared by combining polymer andceramic precursor, and (ii) annealing/calcining the electrospunnanofiber (e.g., to carbonize and remove the polymer and convert theceramic precursor to ceramic).

FIG. 3 illustrates SEM (top) and TEM (bottom) images of the nanofiberscomprising a polymer matrix with ceramic inclusions.

FIG. 4 (panel A) illustrates cycling performance of the capacity ofexemplary ceramic/polymer nanofiber separators provided herein. FIG. 4(panel B) illustrates rate performance over many cycles. Polymer-CeramicNF Separators exhibit higher capacity and much better stability overcycles than a commercial polyethylene (PE) separator.

FIG. 5 illustrates that the cycling performance of the capacity ofhalf-cells with the ceramic-polymer NF separator exhibits highercapacity (10 to 20%) and better stability over cycles than that with acommercial PE separator.

FIG. 6 illustrates that electrochemical impedance spectroscopy (EIS)tests of ceramic-polymer nanofiber separators demonstrate that suchseparators exhibit much lower charge transport resistance and muchhigher Li+ diffusion rate than commercial PE separators.

FIG. 7 illustrates SEM images of nanofibers having an integratedpolymer/ceramic (silica/PEO) matrix.

FIG. 8 illustrates SEM images of silica/PEO nanofibers.

FIG. 9 illustrates SEM images of silica/m-aramid nanofibers.

DETAILED DESCRIPTION OF THE INVENTION

Provided herein are ceramic containing nanofibers and nanofiber mats andprocesses for preparing ceramic containing nanofibers, nanocompositenanofibers and nanofiber mats. In some embodiments, a nanofiber (e.g.,of a plurality of nanofibers, of a nanofiber mat, or of a processdescribed herein) comprises a ceramic material. In some embodiments, theceramic material forms a continuous matrix within the nanofiber. In someembodiments, the nanofiber comprises a ceramic material forming acontinuous matrix within the fiber and a second material, e.g., a secondceramic forming a continuous matrix within the nanofiber. In otherspecific embodiments, the ceramic material forms a plurality of discretedomains within the nanofiber. In more specific embodiments, thenanofiber comprises a ceramic material, forming a plurality of discretedomains within the nanofiber, and a second material, such as a secondceramic material, that forms a continuous matrix within the nanofiber(e.g., provides the continuous structure of the nanofiber). In someembodiments, the nanofiber comprises a ceramic material and a polymermaterial. In specific embodiments, the ceramic material and the polymermaterial both form continuous matrices within the nanofiber. In certainembodiments, the ceramic material forms a continuous core matrix and thepolymer material forms a continuous shell material. In certainembodiments, (e.g., provided by electrospinning a low temperature curingceramic precursor with a polymer and subsequent curing of the ceramicprecursor) the ceramic and polymer materials form an integrated matrix.

In some embodiments, nanofibers provide herein are coaxially layerednanofibers, the nanofibers comprising a core and a sheath that at leastpartially surrounds the core. In some embodiments, the sheath runs alongthe entire length of the nanofiber. In other embodiments, the sheathruns along at least a portion of the nanofiber. In certain embodiments,the core comprises a first material (e.g., a first ceramic material) andthe sheath comprises a second material (e.g., a second ceramicmaterial). In specific embodiments, the first ceramic material isdifferent from the second ceramic material. In other embodiments, one ofthe sheath or core materials is a ceramic and the other is not.

FIG. 1 illustrates a nanofiber provided herein comprising a first and asecond continuous matrix material, wherein the first and secondcontinuous matrix materials are coaxially layered. In specificembodiments, the first material forms the core 110 of the coaxiallylayered nanofiber 107 (illustrated in the cross sectional view 111) andthe second material forms a layer 109 at least partially surrounding thecore 110. In other specific embodiments, the second material forms thecore 110 of the coaxially layered nanofiber 107 (illustrated in thecross sectional view 111) and the first material forms a layer 109 atleast partially surrounding the core 110. In some instances, thenanofibers are prepared by coaxially electrospinning the two layers witha third coaxial layer 108. In some embodiments, the third coaxial layer108 comprises a third matrix material. In other embodiments, the thirdcoaxial layer 108 comprises air, e.g., for gas assisting theelectrospinning process. Moreover, in some embodiments, the core 110 isoptionally hollow, with one or both of the outer layers 109 and/or 108comprising a ceramic material. In some embodiments, provided herein is aceramic containing nanofiber (e.g., a treated nanofiber 107 having atleast two layers, such as illustrated by the cross sectional view 111).In some embodiments, the core layer 110 is a ceramic, the sheath layer109 is polymer (e.g., formed by depositing polymer on a ceramicnanofiber), and the outer layer 108 is absent. In certain embodiments,the core layer 110 comprises a first material (e.g., a ceramic), theintermediate layer 109 comprises a second material—a ceramic, and theouter layer 108 comprises a polymer. FIG. 1 also illustrates anexemplary system or schematic of a process described herein,particularly a system or process for preparing a coaxially layerednanocomposite nanofiber (e.g., by a coaxial gas assisted electrospinningprocess). In some instances, a first fluid stock 101 (e.g., comprising aceramic precursor and a polymer) is electrospun with a second fluidstock 102 (e.g., comprising a second ceramic precursor and a secondpolymer, the second precursor and polymer independently being either thesame or different from the first), and a third fluid (e.g., gas or thirdfluid stock) 103. The fluid stocks may be provided to an electrospinningapparatus by any device, e.g., by a syringe 105. And a gas may beprovided from any source 106 (e.g., air pump). In some embodiments sucha system comprises a plurality of co-axial needles 104. Similarly, 111is representative of an exemplary cross section of coaxialneedles/spinnerets. For example, exemplary co-axial needles comprise anouter sheath tube (which would be represented by 108) at least oneintermediate tube (which is optionally absent, which would berepresented by 109), and a core tube (which would be represented by110). In specific embodiments, such tubes are aligned along a commonaxis (e.g., aligned within 5 degrees of one another). In some instances,the tubes are slightly offset, but the angle of the tubes issubstantially aligned (e.g., within 5 degrees of one another).

In certain embodiments, continuous matrix materials of any nanofiberdescribed herein is continuous over at least a portion of the length ofthe nanofiber. In some embodiments, the continuous matrix material runsalong at least 10% the length of the nanofiber (e.g., on average for aplurality of nanofibers). In more specific embodiments, the continuousmatrix material runs along at least 25% the length of the nanofiber(e.g., on average for a plurality of nanofibers). In still more specificembodiments, the continuous matrix runs along at least 50% the length ofthe nanofiber (e.g., on average for a plurality of nanofibers). In yetmore specific embodiments, the continuous matrix runs along at least 75%the length of the nanofiber (e.g., on average for a plurality ofnanofibers). In some embodiments, the continuous matrix is found alongat least 50%, at least 60%, at least 70%, at least 80%, at least 90%, atleast 95%, at least 98%, or at least 99% the length of the nanofiber(e.g., on average for a plurality of nanofibers). In some embodiments,the continuous matrix material runs along at least 1 micron of thelength of the nanofiber (e.g., on average for a plurality ofnanofibers). In more specific embodiments, the continuous matrixmaterial runs along at least 10 microns of the length of the nanofiber(e.g., on average for a plurality of nanofibers). In still more specificembodiments, the continuous matrix runs along at least 100 microns ofthe length of the nanofiber (e.g., on average for a plurality ofnanofibers). In yet more specific embodiments, the continuous matrixruns along at least 1 mm of the length of the nanofiber (e.g., onaverage for a plurality of nanofibers).

In some embodiments, a nanofiber provide herein comprises discretedomains within the nanocomposite nanofiber. In specific embodiments, thediscrete domains comprise a ceramic material. In certain embodiments,the discrete domains are non-aggregated. In some embodiments, thenon-aggregated domains are dispersed, e.g., in a substantially uniformmanner, along the length of the nanofiber. In certain embodiments, thenanocomposite nanofibers provided herein do not comprise a concentrationof domains in one segment (e.g., a 500 nm, 1 micron, 1.5 micron, 2micron) segment that is over 10 times (e.g., 20 times, 30 times, 50times, or the like) as concentrated as an immediately adjacent segment.In some embodiments, the segment size for such measurements is a definedlength (e.g., 500 nm, 1 micron, 1.5 micron, 2 micron). In otherembodiments, the segment size is a function of the average domain (e.g.,particle) size (e.g., the segment 5 times, 10 times, 20 times, 100 timesthe average domain size). In some embodiments, the domains have a(average) size 1 nm to 1000 nm, 1 nm to 500 nm, 1 nm to 200 nm, 1 nm to100 nm, 20 nm to 30 nm, 1 nm to 20 nm, 30 nm to 90 nm, 40 nm to 70 nm,15 nm to 40 nm, or the like.

Ceramic Material

In various embodiments, the ceramic material in a nanofiber ornanocomposite nanofiber provided herein is any suitable ceramicmaterial. In some embodiments, the ceramic material is an alloy (e.g., ametal oxide that comprises one or more metal), or a ceramic precursor(e.g., aluminum acetate or silicon acetate). In certain embodiments, theceramic material is a material from a ceramic precursor curable at lowtemperature (e.g., room temperature) (e.g. perhydropolysilazane). Inspecific embodiments, the low temperature curing ceramic precursor is apolysilazane, e.g., represented by the formula—[R¹R²SiNR³]_(n)—, whereineach of R¹, R², and R³ are independently selected from H and alkyl(e.g., C₁-C₆ alkyl—i.e., a carbon having 1-6 carbon atoms, or C₁-C₃alkyl), and n is an integer, such as an integer greater than 10 (e.g.,on average), or greater than 100 (e.g., on average). In someembodiments, n is 10-10,000, or 10-1000. In specific embodiments, R³ isH. In more specific embodiments, each of R¹, R², and R³ is H. In certainembodiments, the ceramic material is a material suitable for use in alithium ion battery separator. In some embodiments, the ceramic materialis a precursor material capable of being converted into a materialsuitable for use in a lithium ion battery separator (e.g., a ceramicprecursor). In some embodiments, the ceramic material is a metal oxidecomprising at least one metal (e.g., silicon and/or aluminum).

In certain embodiments, provided herein are nanofibers comprising one ormore ceramic material. In some embodiments, the nanofibers comprise atleast 3% by weight of the ceramic material. In specific embodiments, thenanofibers comprise at least 9% by weight of the ceramic material. Inmore specific embodiments, the nanofibers comprise at least 25% byweight of the ceramic material (e.g., on average for a plurality ofnanofibers). In more specific embodiments, the nanofibers comprise atleast 50% by weight of the ceramic material (e.g., on average for aplurality of nanofibers). In still more specific embodiments, thenanofibers comprise at least 75% by weight of the ceramic material(e.g., on average for a plurality of nanofibers). In yet more specificembodiments, the nanofibers comprise at least 90% by weight of theceramic material (e.g., on average for a plurality of nanofibers). Inspecific embodiments, the nanofibers comprise at least 95% by weight ofthe ceramic material (e.g., on average for a plurality of nanofibers).

Any suitable ceramic material is optionally utilized. Preferably, suchceramic materials are inert—or substantially inert—in a battery (e.g.,lithium ion battery) environment (e.g., under normal and runawayconditions). In some embodiments, the ceramic material(s) comprisessilica, alumina, zirconia, beryllia, ceria, titania, barium titanate,strontium titanate, or the like, or combinations thereof.

Second Material

In some embodiments, a nanocomposite nanofiber provided herein comprisesa ceramic material (or a ceramic precursor) and a second material. Incertain embodiments, additional materials are optionally present. Insome embodiments, the second material is a continuous matrix material,as described herein. In certain embodiments, the second material is asecond ceramic or a polymer (e.g., when the ceramic material is aceramic precursor).

In some embodiments, provided herein is a nanocomposite nanofibercomprising a first material and a second material. In certainembodiments, provided herein is a nanocomposite ceramic nanofibercomprising a first ceramic material and a second material (e.g., aceramic material or a polymer material). In certain embodiments,provided herein is a nanocomposite ceramic nanofiber comprising a firstceramic material and a second ceramic material. In some embodiments, thefirst material is a first continuous ceramic matrix material. Inspecific embodiments, the first (ceramic) material is a first continuousmatrix material and the second (e.g., ceramic) material is a secondcontinuous matrix material. In more specific embodiments, the first(ceramic) material forms the core of a coaxially layered nanocompositenanofiber and the second (e.g., ceramic) material forms the sheath atleast partially surrounding the core. In certain embodiments, suchnanocomposite nanofibers optionally comprise an additional matrixmaterial between the ceramic containing core and ceramic containingsheath, and/or an additional (e.g., matrix) material on the surface ofthe ceramic containing sheath (e.g., at least partially surrounding theceramic containing sheath). In some embodiments, when the nanofibercomprises a polymer material as a second material, the polymer isoptionally deposited on the nanofiber, e.g., by dissolving the polymerin a solution and exposing a ceramic nanofiber to the solution—e.g.,followed by evaporation of solvent.

In some embodiments, a nanocomposite nanofiber provided herein comprisesa first material and a second material, the first and second materialsforming an integrated matrix (e.g., the materials are in the same layerand are well dispersed along the length of the nanofiber—in someinstances one or both of the integrated materials individually form acontinuous matrix in the nanofiber). In some embodiments, the firstmaterial is a ceramic and the second material is a ceramic. In otherembodiments, the first material is a ceramic and the second material isa polymer. In specific embodiments, the first material is silica and thesecond material is a polymer (e.g., PEO).

In some embodiments, a nanocomposite nanofiber provided herein comprisesa first material and a second material, the first material comprising apolymer and forming a continuous matrix and the second materialcomprising a ceramic precursor (e.g., a metal salt, such as siliconacetate, zirconium acetate, or a low temperature curing ceramicprecursor, such as perhydropolysilazane). In specific embodiments, theceramic precursor is a silica precursor (e.g., a low temperature curingceramic precursor, such as a polysilazane—e.g., perhydropolysilazane).

In certain embodiments, the polymer material is or comprisespolyisoprene (PI), a polylactic acid (PLA), a polyvinyl alcohol (PVA), apolyethylene oxide (PEO), a polyvinylpyrrolidone (PVP), polyacrylamide(PAA), polyacrylonitrile (PAN), or any combination thereof.

Polymer Material

In some embodiments, a polymer in a process, fluid stock or nanofiberdescribed herein is an organic polymer. In some embodiments, polymersused in the compositions and processes described herein are hydrophilicpolymers, including water-soluble and water swellable polymers. In someaspects, the polymer is soluble in water, meaning that it forms asolution in water. In other embodiments, the polymer is swellable inwater, meaning that upon addition of water to the polymer the polymerincreases its volume up to a limit. Exemplary polymers suitable for thepresent methods and compositions include but are not limited topolyvinyl alcohol (“PVA”), polyvinyl acetate (“PVAc”), polyethyleneoxide (“PEO”), polyvinyl ether, polyvinyl pyrrolidone, polyglycolicacid, hydroxyethylcellulose (“HEC”), ethylcellulose, cellulose ethers,polyacrylic acid, polyisocyanate, aramid, and the like. In someembodiments, the polymer is isolated from biological material. In someembodiments, the polymer is starch, chitosan, xanthan, agar, guar gum,and the like. In other instances, other polymers, such aspolyacrylonitrile (“PAN”) are optionally utilized (e.g., with DMF as asolvent in electrospinning or other processes). In other instances, apolyacrylate (e.g., polyalkacrylate, polyacrylic acid,polyalkylalkacrylate, such as poly(methyl methacrylate) (PMMA), or thelike), or polycarbonate is optionally utilized. In some instances, thepolymer is polyacrylonitrile (PAN), polyvinyl alcohol (PVA), apolyethylene oxide (PEO), polyvinylpyridine, polyisoprene (PI),polyimide, polylactic acid (PLA), a polyalkylene oxide, polypropyleneoxide (PPO), polystyrene (PS), a polyarylvinyl, a polyheteroarylvinyl, anylon, a polyacrylate (e.g., poly acrylic acid,polyalkylalkacrylate—such as polymethylmethacrylate (PMMA),polyalkylacrylate, polyalkacrylate), polyacrylamide,polyvinylpyrrolidone (PVP) block, polyacrylonitrile (PAN), polyglycolicacid, hydroxyethylcellulose (HEC), ethylcellulose, cellulose ethers,polyacrylic acid, polyisocyanate, or a combination thereof.

In certain embodiments, a polymer provided herein has any suitablemolecular weight, e.g., at least 50,000 g/mol, at least 100,000 g/mol,at least 500,000 g/mol or the like. In some embodiments, the molecularweight is 50,000 to 1,000,000 g/mol.

Nanofibers

In certain embodiments, nanofiber provided herein have any suitablecharacteristic.

In some embodiments, a nanofiber provided herein has a diameter of lessthan 2 microns (e.g., an average diameter of a plurality of nanofibers).In specific embodiments, a nanofiber provided herein has a diameter ofless than 1.5 microns (e.g., an average diameter of a plurality ofnanofibers). In more specific embodiments, a nanofiber provided hereinhas a diameter of less than 1 micron (e.g., an average diameter of aplurality of nanofibers). In still more specific embodiments, ananofiber provided herein has a diameter of less than 850 nm (e.g., anaverage diameter of a plurality of nanofibers). In yet more specificembodiments, a nanofiber provided herein has a diameter of less than 750nm (e.g., an average diameter of a plurality of nanofibers). In morespecific embodiments, a nanofiber provided herein has a diameter of lessthan 600 nm (e.g., an average diameter of a plurality of nanofibers). Insome embodiments, a nanofiber provided herein has a diameter of at least50 nm. In specific embodiments, a nanofiber provided herein has adiameter of at least 100 nm. In still more specific embodiments, ananofiber provided herein has a diameter of at least 200 nm.

In some embodiments, nanofibers provided herein have a (e.g., average)length of at least 1 μm, at least 10 μm, at least 20 μm, at least 100μm, at least 500 μm, at least 1,000 μm, at least 5,000 μm, at least10,000 μm, or the like. In specific embodiments, nanofibers providedherein have a (e.g., average) length of at least 1 mm.

In some embodiments, a nanofiber provided herein has an aspect ratio ofgreater than 10 (e.g., an average aspect ratio of a plurality ofnanofibers). In specific embodiments, a nanofiber provided herein has anaspect ratio of greater than 100 (e.g., an average aspect ratio of aplurality of nanofibers). In more specific embodiments, a nanofiberprovided herein has an aspect ratio of greater than 500 (e.g., anaverage aspect ratio of a plurality of nanofibers). In still morespecific embodiments, a nanofiber provided herein has an aspect ratio ofgreater than 1000 (e.g., an average aspect ratio of a plurality ofnanofibers). In yet more specific embodiments, a nanofiber providedherein has an aspect ratio of greater than 10⁴ (e.g., an average aspectratio of a plurality of nanofibers).

In some embodiments, nanofibers provided herein comprise (e.g., onaverage) at least 99%, at least 98%, at least 97%, at least 96%, atleast 95%, at least 90%, at least 80%, or the like of metal and oxygen,when taken together, by mass (e.g., elemental mass). In someembodiments, nanofibers provided herein comprise (e.g., on average) atleast 99%, at least 98%, at least 97%, at least 96%, at least 95%, atleast 90%, at least 80%, or the like of metal, carbon and oxygen, whentaken together, by mass (e.g., elemental mass).

In some embodiments, the porosity of a nanofiber mat (comprising one ormore nanofiber described herein) is at least 5%, at least 10%, at least15%, at least 20%, at least 25%, at least 50%, or the like. Porosity canbe measured in any suitable manner. For example, in some instances, theporosity of a nanofiber mat is determined by measuring the fluid volumepresent in the nanofiber mat after the nanofiber mat is submerged in orfilled with a fluid.

Described herein are nanofibers and methods for making nanofibers thathave a plurality of pores. The pores may be of any suitable size orshape. In some embodiments the pores are “mesopores”, having a diameterof less than 100 nm (e.g., between 2 and 50 nm, on average). In someembodiments, the pores are “ordered”, such as having a substantiallyuniform shape, a substantially uniform size and/or are distributedsubstantially uniformly through the nanofiber. In some embodiments,nanofibers described herein have a high surface area and/or specificsurface area (e.g., surface area per mass of nanofiber and/or surfacearea per volume of nanofiber). In some embodiments, nanofibers describedherein comprise ordered pores, e.g., providing substantially flexibleand/or non-brittleness.

In one aspect, described herein are nanofibers comprising any one ormore of: (a) a surface area of at least 10 π r h, wherein r is theradius of the nanofiber and h is the length of the nanofiber; (b) aspecific surface area of at least 100 m²/g; (c) a porosity of at least20% and a length of at least 1 μm; (d) a porosity of at least 35%,wherein the nanofiber is substantially contiguous; (e) a porosity of atleast 35%, wherein the nanofiber is substantially flexible ornon-brittle; (f) a plurality of pores with an average diameter of atleast 1 nm; (g) a plurality of pores, wherein the pores have asubstantially uniform shape; (h) a plurality of pores, wherein the poreshave a substantially uniform size; and (i) a plurality of pores, whereinthe pores are distributed substantially uniformly throughout thenanofiber.

In some embodiments, the pores comprise spheres, cylinders, layers,channels, or any combination thereof. In some embodiments, the pores arehelical. In some embodiments, the nanofiber comprises metal, metalalloy, ceramic, polymer, or any combination thereof.

In one aspect, described herein is a method for producing an orderedmesoporous nanofiber, the method comprising: (a) coaxiallyelectrospinning a first fluid stock with a second fluid stock to producea first nanofiber, the first fluid stock comprising at least one blockco-polymer and a ceramic component (e.g., ceramic precursor), the secondfluid stock comprising a coating agent, and the first nanofibercomprising a first layer (e.g., core) and a second layer (e.g., coat)that at least partially coats the first layer; (b) annealing the firstnanofiber; (c) optionally removing the second layer from the firstnanofiber to produce a second nanofiber comprising the block co-polymer;and (d) selectively removing at least part of the block co-polymer fromthe first nanofiber or the second nanofiber (e.g. thereby producing anordered mesoporous nanofiber). Additional coaxial layers areoptional—e.g., comprising a precursor and block copolymer for anadditional mesoporous layer, or a precursor and a polymer as describedherein for a non-mesoporous layer.

In some embodiments, the block co-polymer comprises a polyisoprene (PI)block, a polylactic acid (PLA) block, a polyvinyl alcohol (PVA) block, apolyethylene oxide (PEO) block, a polyvinylpyrrolidone (PVP) block,polyacrylamide (PAA) block or any combination thereof (i.e., thermallyor chemically degradable polymers).

In some embodiments, the block co-polymer further comprises a block thatdoes not degrade under conditions suitable for degrading and/or removingthe degradable and/or removable block.

In some embodiments, the block co-polymer comprises a polystyrene (PS)block, a poly(methyl methacrylate) (PMMA) block, a polyacrylonitrile(PAN) block, or any combination thereof (i.e., thermally or chemicallystable polymers).

In some embodiments, the coating layer and at least part of the blockco-polymer (concurrently or sequentially) is selectively removed in anysuitable manner, such as, by heating, by ozonolysis, by treating with anacid, by treating with a base, by treating with water, by combinedassembly by soft and hard (CASH) chemistries, or any combinationthereof.

Additionally, U.S. Application Ser. No. 61/599,541 is incorporatedherein by reference for disclosures related to such techniques.

Batteries and Separators

In some embodiments, provided herein is a battery (e.g., a primary orsecondary cell) comprising at least one nanofiber described herein. Inspecific instances, the battery comprises plurality of such nanofibers,e.g., a non-woven mat thereof. In some embodiments, the batterycomprises at least two electrodes (e.g., an anode and a cathode) and aseparator, the separator comprising at least one nanofiber describedherein. In specific embodiments, the battery is a lithium-ion batteryand the separator comprises at least one nanofiber described herein(e.g., a nanofiber mat thereof). Likewise, provided herein is a batteryseparator comprising any nanocomposite nanofiber described herein (e.g.,a nanofiber mat comprising one or more such nanofibers).

In certain embodiments, separator nanofibers (e.g., mat thereof) arecompressed—at any suitable pressure for any suitable amount of time. Insome embodiments, a process described herein comprises compressing thenanofibers (e.g., electrospinning or assembling a non-woven mat ofnanofibers and subsequently compressing the non-woven mat). In someembodiments, the nanofibers are compressed at a pressure of 0.1 Mpa to10 Mpa. In some embodiments, the nanofibers are compressed at a pressureof 1 Mpa to 5 Mpa.

In further or alternative embodiments, the separator has any suitablethickness, such as a thickness of 10-500 micron. In some embodiments,the separator has a thickness of 10-200 micron. In specific embodiments,the separator has a thickness of 15-100 micron.

FIG. 4 (panel A) illustrates cycling performance of the capacity ofexemplary ceramic/polymer nanofiber separators provided herein. FIG. 4(panel B) illustrates rate performance over many cycles. Polymer-CeramicNF Separators exhibit higher capacity and much better stability overcycles than a commercial polyethylene (PE) separator. In someembodiments, separators provided herein provide discharge capacitiesafter 50 cycles that are at least 50% of the initial discharge capacitywhen provided in a half-cell with a LiCoO₂ cathode (as illustrated inFIG. 4—panel A) (and Li anode). In certain embodiments, separatorsprovided herein provide discharge capacities after 70 cycles that are atleast 50% of the initial discharge capacity when provided in such ahalf-cell. In some embodiments, separators provided herein providedischarge capacities after 50 cycles that are at least 75% of theinitial discharge capacity. In certain embodiments, separators providedherein provide discharge capacities after 70 cycles that are at least75% of the initial discharge capacity. In some embodiments, separatorsprovided herein provide discharge capacities after 50 cycles that are atleast 25% of the initial discharge capacity. In certain embodiments,separators provided herein provide discharge capacities after 70 cyclesthat are at least 25% of the initial discharge capacity. In certainembodiments, separators provided herein provide a discharge capacity ofat least 100 mAh/g in a lithium ion half-cell with a LiCoO₂ cathode (andLi anode) following the cycle parameters of FIG. 4 (panel B) (i.e., 0.1C, 0.17 C, 0.37 C, 1.0 C, and repeating)—e.g., on the second 0.37 Ccycle. In some embodiments, the discharge capacity under similarconditions is at least 60 mAh/g on the second 1.0 C cycle. In further oralternative embodiments, the discharge capacity is at least 120 mAh/g onthe first 1.0 C cycle. In specific embodiments, the discharge capacitiesare at least as those set forth in FIG. 4 (panel B), or at least 90%thereof (in each cycle).

FIG. 5 illustrates that the cycling performance of the capacity ofhalf-cells with the ceramic-polymer NF separator exhibits highercapacity (10 to 20%) and better stability over cycles than that with acommercial PE separator. FIG. 6 illustrates that electrochemicalimpedance spectroscopy (EIS) tests of ceramic-polymer nanofiberseparators demonstrate that such separators exhibit much lower chargetransport resistance and much higher Li+ diffusion rate than commercialPE separators. In some embodiments, the transport resistance of aseparator described herein is equal to or less than that illustrated inFIG. 6. In certain embodiments, the lithium ion diffusion rate of aseparator provided herein is at least as great as that illustrated inFIG. 6.

Process

In certain embodiments, provided herein is a process for preparingceramic-containing nanofibers. In some embodiments, suchceramic-containing nanofibers (e.g., nanocomposite nanofibers) comprisehigh amounts of ceramic (e.g., as described herein). Moreover, in someembodiments, provided herein are high quality nanofibers and processesfor preparing high quality nanofibers that have good structuralintegrity, few voids, few structural defects, tunable length, and thelike. In certain embodiments, high loading of precursor, relative topolymer loading, in the fluid stock and/or precursor/electrospunnanofibers, facilitates and/or provides such high quality nanofibers. Ingeneral, the processes described herein provide the ability to preparenanostructures with improved performance properties over othernanostructures, such as those prepared by nanowire growth, includingdeposition, precipitation and growth techniques.

In some embodiments, the electrospun (e.g., as-spun) nanofibercomprising a ceramic material and a polymer is prepared by electrospinning a fluid stock, the fluid stock comprising (1) a ceramiccomponent (e.g., ceramic precursor); and (2) polymer. In specificembodiments, the nanofiber comprises ceramic precursor and polymer. Inother specific embodiments, the nanofiber comprises ceramic and polymer.

In some embodiments, provided herein is a process for preparing aceramic-containing nanofiber (e.g., for use as or in a batteryseparator, or any other suitable application), the process comprising:

-   -   a. electrospinning a fluid stock, the fluid stock comprising or        prepared by combining (i) a ceramic component (e.g., ceramic        precursor, ceramic inclusions—such as nano-inclusions, e.g.,        nanoparticles), to produce a first nanofiber (e.g., electrospun        or as-spun nanofiber); and    -   b. annealing the first nanofiber to produce a ceramic-containing        nanofiber.

In specific embodiments, the ceramic component is a ceramic precursor,such as any precursor described herein (e.g., a metal acetate, metalhalide, metal diketone, or the like—which in the fluid stock isoptionally partially or completely associated with the polymer). In morespecific embodiments, the ceramic component is a low temperature curingceramic precursor (e.g., a polysilazane, such as perhydropolysilazane).

In some embodiments, e.g., if a high temperature calcining or curingceramic precursor is utilized (e.g., which may result in carbonizationand/or removal of the polymer), the process further comprises depositingpolymer on the ceramic-containing nanofiber. In specific embodiments,such deposition is achieved in any suitable manner, such as by exposingthe ceramic-containing nanofibers to a polymer solution, byelectrospraying polymer onto the ceramic-containing nanofibers, or thelike. In other embodiments, e.g., wherein a low-temperature curingceramic precursor is utilized, such a step is not necessary, as thepolymer utilized in the electrospinning process need not be removedduring curing of the ceramic precursor to ceramic.

In certain embodiments, the ceramic-containing nanofiber is optionallyany ceramic-containing nanofiber described herein, e.g., nanofiberscomprising a continuous matrix of ceramic, nanofibers comprising acontinuous matrix of ceramic and a continuous matrix of polymer,nanofibers comprising an integrated matrix of ceramic and polymer, orthe like. In some embodiments, a nanofiber described herein comprises anintegrated matrix of ceramic and polymer, the ceramic being a porous(e.g., mesoporous) ceramic matrix comprising polymer positioned within(e.g., within at least a portion of) the porous structures of theceramic matrix.

In specific embodiments, the fluid stock of any process or compositionprovided herein comprises an aqueous medium (e.g., water or an aqueousmixture, such as water/alcohol, water/acetic acid, or the like). Inother embodiments, the fluid stock comprises an organic solvent (e.g.,dimethylformamide (DMF) when the polymer is PAN), or a polymer melt.

In some embodiments, the processes further comprises a treatment step orprocess (e.g., of the electrospun (e.g., as-spun, or pre-treated, suchas with low temperature annealing or washing). In some embodiments, thetreatment process comprises (a) thermal treatment; (b) chemicaltreatment; or (c) a combination thereof. In specific embodiments,treatment of the electrospun (e.g., as-spun) nanofiber comprisesthermally treating the electrospun (e.g., as-spun) nanofiber underoxidative conditions (e.g., air)—exemplary chemical treatment. In otherspecific embodiments, treatment of the as-spun nanofiber comprisesthermally treating the as-spun nanofiber under inert conditions (e.g.,argon). In still other specific embodiments, treatment of the as-spunnanofiber comprises thermally treating the as-spun nanocompositenanofiber (e.g., comprising polymer and ceramic component) underreducing conditions (e.g., hydrogen, or a hydrogen/argon blend). Incertain embodiments, the as-spun nanofiber is heated to a temperature ofabout 500° C. to about 2000° C., at least 900° C., at least 1000° C., orthe like. In specific embodiments, the as-spun nanofiber is heated to atemperature of about 1000° C. to about 1800° C., or about 1000° C. toabout 1700° C. In other embodiments, thermal treatment does not requireelevated temperatures. For example, in examples where a low temperatureannealing ceramic precursor is utilized, thermal treatment optionallyoccurs at low temperature (e.g., below 50° C., or room temperature).

In one aspect, the process has a high yield (e.g., which is desirablefor embodiments in which the precursor is expensive). In someembodiments, the metal atoms in the nanofiber are about 10%, about 20%,about 30%, about 33%, about 40%, about 50%, about 60%, about 70%, about80%, about 90%, about 95%, about 98%, or about 100% of the number of(e.g., in moles) metal molecules in the fluid stock.

In some embodiments, the fluid stock is uniform or homogenous. Inspecific embodiments, the process described herein comprises maintainingfluid stock uniformity or homogeneity. In some embodiments, fluid stockuniformity and/or homogeneity is achieved or maintained by any suitablemechanism, e.g., by agitating, heating, or the like. Methods ofagitating include, by way of non-limiting example, mixing, stirring,shaking, sonicating, or otherwise inputting energy to prevent or delaythe formation of more than one phase in the fluid stock.

In some embodiments, (e.g., where ceramic precursors are utilized, suchas a metal salts, metal complexes, or other metal-ligand associations)the weight ratio of the ceramic component(s) (including one or moreceramic precursors) to polymer is at least 1:3, at least 1:2, at least1:1, at least 1.25:1, at least 1.5:1, at least 1.75:1, at least 2:1, atleast 3:1, or at least 4:1. In certain embodiments, e.g., whereinceramic nano-inclusions are utilized, the ceramic component to polymerratio (e.g., in the fluid stock, or a polymer/ceramic nanofiber providedherein) is at least 1:30, at least 1:20, at least 1:10, or the like. Inspecific embodiments, the ceramic component to polymer ratio is 1:30 to5:1, e.g., 1:25 to 1:1. Or, more specifically, 1:15 to 1:2. In someembodiments, the monomeric residue (i.e., repeat unit) concentration ofthe polymer in the fluid stock is at least 100 mM. In specificembodiments, the monomeric residue (i.e., repeat unit) concentration ofthe polymer in the fluid stock is at least 200 mM. In more specificembodiments, the monomeric residue (i.e., repeat unit) concentration ofthe polymer in the fluid stock is at least 400 mM. In still morespecific embodiments, the monomeric residue (i.e., repeat unit)concentration of the polymer in the fluid stock is at least 500 mM. Insome embodiments, the fluid stock comprises at least about 0.5 weight %,at least about 1 weight %, at least about 2 weight %, at least about 5weight %, at least about 10 weight %, or at least about 20 weightpolymer.

In some embodiments, the ceramic precursor comprises a metal salt, metalcomplex, or any other suitable metal-ligand association. In specificembodiments, the ceramic precursor comprises a silicon precursor,aluminum precursor, zirconium precursor, beryllium precursor, ceriumprecursor, barium precursor, strontium precursor, titanium precursor, orthe like, or a combination thereof. In specific embodiments, ceramicprecursors include metal salts or complexes, wherein the metal isassociated with any suitable anion or other Lewis Base, e.g., acarboxylate (e.g., —OCOCH₃ or another—OCOR group, wherein R is an alkyl,substituted alkyl, aryl, substituted aryl, or the like), an alkoxide(e.g., a methoxide, ethoxide, isopropyl oxide, t-butyl oxide, or thelike), a halide (e.g., chloride, bromide, or the like), a diketone(e.g., acetylacetone, hexafluoroacetylacetone, or the like), a nitrates,amines (e.g., NR′₃, wherein each R″ is independently R or H or two R″,taken together form a heterocycle or heteroaryl), and combinationsthereof.

In some embodiments, a ceramic inclusion (e.g., nano-inclusion, such asnanoparticle) comprises any suitable ceramic, such as silica, alumina,zirconia, beryllia, ceria, titania, barium titanate, strontium titanate,bentonite, or the like, or combinations thereof.

In some embodiments, a polymer in a process or nanofiber describedherein is an organic polymer. In some embodiments, polymers used in thecompositions and processes described herein are hydrophilic polymers,including water-soluble polymers. In some aspects, water-solublepolymers include polymers that are dissolvable and swellable in water.Exemplary polymers suitable for the present methods include but are notlimited to polyvinyl alcohol (“PVA”), polyvinyl acetate (“PVAc”),polyethylene oxide (“PEO”), polyvinyl ether, polyvinyl pyrrolidone,polyglycolic acid, hydroxyethylcellulose (“HEC”), ethylcellulose,cellulose ethers, polyacrylic acid, polyisocyanate, and the like. Insome embodiments, the polymer is isolated from biological material. Insome embodiments, the polymer is starch, chitosan, xanthan, agar, guargum, and the like.

In some embodiments, a polymer described herein (e.g., in a process,precursor nanofiber, a fluid stock, or the like) is a polymer (e.g.,homopolymer or copolymer) comprising a plurality of reactive sites. Incertain embodiments, the reactive sites are nucleophilic (i.e., anucleophilic polymer) or electrophilic (i.e., an electrophilic polymer).For example, in some embodiments, a nucleophilic polymer describedherein comprises a plurality of alcohol groups (such as polyvinylalcohol—PVA—or a cellulose), ether groups (such as polyethyleneoxide—PEO—or polyvinyl ether—PVE), and/or amine groups (such aspolyvinyl pyridine, ((di/mono)alkylamino)alkyl alkacrylate, or thelike).

In certain embodiments, the polymer is a nucleophilic polymer (e.g., apolymer comprising alcohol groups, such as PVA). In some embodiments,the polymer is a nucleophilic polymer and a ceramic precursor is anelectrophilic precursor (e.g., a metal acetate, metal chloride, or thelike). In specific embodiments, the nucleophilic polymer and the ceramicprecursor form a precursor-polymer association in the fluid stock and/orthe as-spun nanofiber and that association is a reaction product betweena nucleophilic polymer and electrophilic precursor(s).

In other embodiments, the polymer is an electrophilic polymer (e.g., apolymer comprising chloride or bromide groups, such as polyvinylchloride). In some embodiments, the polymer is an electrophilic polymerand a precursor (e.g., ceramic precursor) is a nucleophilic precursor(e.g., metal-ligand complex comprising “ligands” with nucleophilicgroups, such as alcohols or amines). In specific embodiments, thenucleophilic polymer and the ceramic precursor form a precursor-polymerassociation in the fluid stock and/or the as-spun nanofiber and thatassociation is a reaction product (e.g., forming an ionic or covalentbond) between an electrophilic polymer and a nucleophilic precursor.

For the purposes of this disclosure ceramic precursors include bothpreformed metal-ligand associations (e.g., salts, metal-complexes, orthe like) (e.g., reagent precursors, such as metal acetates, metalhalides, or the like) and/or metal-polymer associations (e.g., as formedfollowing combination of reagent precursor with polymer in an aqueousfluid).

In other embodiments, sol gel electrospinning may optionally be utilizedto prepare ceramic nanofibers described herein. In one exemplaryembodiment, sol gel electrospinning techniques include electrospinning(e.g., in a gas assisted manner) a sol fluid stock is electrospun (e.g.,coaxially for a coaxially layered ceramic nanofiber). In specificembodiments, sol fluid stocks are prepared by combining an inorganicmaterial suitable for forming a sol (e.g., tetraethyl ortho-silicate(TEOS), triethyl phosphate, titanium isopropanol, etc.) with a solvent(e.g., alcohol, water, isopropanol, acetic acid, or the like, dependingon the system) and ripening the combination to form a sol-gel (e.g., byallowing the combination to stand, or by heating the combination—whichmay accelerate the process).

Electrospinning

In some embodiments, the process comprises electrospinning a fluidstock. Any suitable method for electrospinning is used.

In some instances, elevated temperature electrospinning is utilized.Exemplary methods for comprise methods for electrospinning at elevatedtemperatures as disclosed in U.S. Pat. No. 7,326,043 and U.S. Pat. No.7,901,610, which are incorporated herein for such disclosure. In someembodiments, elevated temperature electrospinning improves thehomogeneity of the fluid stock throughout the electrospinning process.

In some embodiments, gas assisted electrospinning is utilized (e.g.,about a common axis with the jet electrospun from a fluid stockdescribed herein). Exemplary methods of gas-assisted electrospinning aredescribed in PCT Patent Application PCT/US2011/024894 (“Electrospinningapparatus and nanofibers produced therefrom”), which is incorporatedherein for such disclosure. In gas-assisted embodiments, the gas isoptionally air or any other suitable gas (such as an inert gas,oxidizing gas, or reducing gas). In some embodiments, gas assistanceincreases the throughput of the process and/or reduces the diameter ofthe nanofibers. In some instances, gas assisted electrospinningaccelerates and elongates the jet of fluid stock emanating from theelectrospinner. In some embodiments, incorporating a gas stream inside afluid stock produces hollow nanofibers. In some embodiments, the fluidstock is electrospun using any suitable method.

In specific embodiments, the process comprises coaxial electrospinning(electrospinning two or more fluids about a common axis). As describedherein, coaxial electrospinning a first fluid stock as described herein(e.g., comprising a ceramic component and a polymer) with a second fluidis used to add coatings, make hollow nanofibers, make nanofiberscomprising more than one material, and the like. In various embodiments,the second fluid is either outside (i.e., at least partiallysurrounding) or inside (e.g., at least partially surrounded by) thefirst fluid stock. In some embodiments, the second fluid is a gas(gas-assisted electrospinning). In some embodiments, gas assistanceincreases the throughput of the process, reduces the diameter of thenanofibers, and/or is used to produce hollow nanofibers. In someembodiments, the method for producing nanofibers comprises coaxiallyelectrospinning the first fluid stock and a gas. In other embodiments,the second fluid is a second fluid stock and comprises a polymer and anoptional ceramic component (e.g., a ceramic precursor).

The term “alkyl” as used herein, alone or in combination, refers to anoptionally substituted straight-chain, or optionally substitutedbranched-chain saturated or unsaturated hydrocarbon radical. Examplesinclude, but are not limited to methyl, ethyl, propyl, butyl, pentyl,hexyl, and longer alkyl groups, such as heptyl, octyl and the like.certain instances, “alkyl” groups described herein include linear andbranched alkyl groups, saturated and unsaturated alkyl groups, andcyclic and acyclic alkyl groups.

The term “aryl” as used herein, alone or in combination, refers to anoptionally substituted aromatic hydrocarbon radical of six to abouttwenty ring carbon atoms, and includes fused and non-fused aryl rings. Anon-limiting example of a single ring aryl group includes phenyl; afused ring aryl group includes naphthyl.

The term “heteroaryl” as used herein, alone or in combination, refers tooptionally substituted aromatic monoradicals containing from about fiveto about twenty skeletal ring atoms, where one or more of the ring atomsis a heteroatom independently selected from among oxygen, nitrogen,sulfur, phosphorous, silicon, selenium and tin but not limited to theseatoms and with the proviso that the ring of the group does not containtwo adjacent O or S atoms. A non-limiting example of a single ringheteroaryl group includes pyridyl; fused ring heteroaryl groups includebenzimidazolyl, quinolinyl, acridinyl.

EXAMPLES Example 1 Preparing a Fluid Stock of Silicon Acetate and PVA

2 grams of silicon acetate, the metal precursor(s), is dissolved in 20ml of 1 molar acetic acid solution. The solution is stirred for 2 hoursto create a solution of silicon acetate.

In a second solution, 1 gram of 99.7% hydrolyzed polyvinyl alcohol (PVA)with an average molecular weight of 79 kDa and polydispersity index of1.5 is dissolved in 10 ml of de-ionized water. The polymer solution isheated to a temperature of 95° C. and stirred for 2 hours to create ahomogenous solution.

The silicon acetate solution is then combined with the PVA solution tocreate a fluid stock. In order to distribute the precursor substantiallyevenly in the fluid stock, the precursor solution is added gradually tothe polymer solution while being continuously vigorously stirred for 2hours. The mass ratio of precursor to polymer for the fluid feed (basedon initial silicon acetate mass) was 2:1.

Example 2 Preparing a Fluid Stock of Aluminum Acetate and PVA

2 grams of aluminum acetate, the metal precursor(s), is dissolved in 20ml of 1 molar acetic acid solution. The solution is stirred for 2 hoursto create a solution of aluminum acetate.

In a second solution, 1 gram of 99.7% hydrolyzed polyvinyl alcohol (PVA)with an average molecular weight of 79 kDa and polydispersity index of1.5 is dissolved in 10 ml of de-ionized water. The polymer solution isheated to a temperature of 95° C. and stirred for 2 hours to create ahomogenous solution.

The aluminum acetate solution is then combined with the PVA solution tocreate a fluid stock. In order to distribute the precursor substantiallyevenly in the fluid stock, the precursor solution is added gradually tothe polymer solution while being continuously vigorously stirred for 2hours. The mass ratio of precursor to polymer for the fluid feed (basedon initial aluminum acetate mass) was 2:1.

Example 3 Preparing Silica Nanofiber

A fluid stock of Example 1 is electrospun in a co-axial manner using anozzle/spinneret similar to the one depicted in FIG. 1 (where 111illustrates the nozzle/spinneret). The center conduit contains siliconacetate fluid stock of Example 1 and the outer conduit contains a gasstream (high velocity/pressurized air)—gas assisted electrospinning. Theouter tube depicted in FIG. 1 is absent. The electrospun nanofiber iscalcinated by heating for 2 hours at 600° C. in an atmosphere of air.

Example 4 Preparing Alumina Nanofiber

A fluid stock of Example 2 is electrospun in a co-axial manner using anozzle/spinneret similar to the one depicted in FIG. 1 (where 111illustrates the nozzle/spinneret). The center conduit contains aluminumacetate fluid stock of Example 2 and the outer conduit contains a gasstream (high velocity/pressurized air)—gas assisted electrospinning. Theouter tube depicted in FIG. 1 is absent. The electrospun nanofiber iscalcinated by heating for 2 hours at 600° C. in an atmosphere of air.

Example 5 Preparing Silica/Alumina Nanocomposite Nanofiber

Two fluid stocks are electrospun in a co-axial manner using a spinneretsimilar to the one depicted in FIG. 1 (where 111 illustrates thenozzle/spinneret). The center conduit contains aluminum acetate fluidstock of Example 2 and the outer conduit contains silicon acetate fluidstock of Example 1. The electrospinning procedure is optionallygas-assisted, e.g., by flowing high velocity gas through the outer tubedepicted in FIG. 1. The electrospun hybrid fluid stock is calcinated byheating for 2 hours at 600° C. in an atmosphere of air.

Example 6 Preparing a Fluid Stock of Zirconium Acetate and PVA

2 grams of zirconium acetate, the metal precursor, is dissolved in 20 mlof 1 molar acetic acid solution. The solution is stirred for 2 hours tocreate a solution of aluminum acetate.

In a second solution, 1 gram of 99.7% hydrolyzed polyvinyl alcohol (PVA)with an average molecular weight of 79 kDa and polydispersity index of1.5 is dissolved in 10 ml of de-ionized water. The polymer solution isheated to a temperature of 95° C. and stirred for 2 hours to create ahomogenous solution.

The zirconium acetate solution is then combined with the PVA solution tocreate a fluid stock. In order to distribute the precursor substantiallyevenly in the fluid stock, the precursor solution is added gradually tothe polymer solution while being continuously vigorously stirred for 2hours. The mass ratio of precursor to polymer for the fluid feed (basedon initial zirconium acetate mass) was 2:1. FIG. 2 illustrates an x-raydiffraction plot of ZrO₂ nanofibers from electrospinning of Zr—Ac/PVA(2:1) solution.

Example 7 Preparing Zirconia Nanofiber

A fluid stock of Example 6 is electrospun in a co-axial manner using anozzle/spinneret similar to the one depicted in FIG. 1 (where 111illustrates the nozzle/spinneret). The center conduit contains zirconiumacetate fluid stock of Example 2 and the outer conduit contains a gasstream (high velocity/pressurized air)—gas assisted electrospinning. Theouter tube depicted in FIG. 1 is absent. The electrospun nanofiber iscalcinated by heating for 2 hours at 800° C. in an atmosphere of air.FIG. 2 illustrates zirconium precursor nanofibers (left) having averagediameters of 800-1000 nm, and zirconia nanofibers (right), havingaverage diameters of 300-600 nm.

Example 8 Preparing Silica/Zirconium Nanocomposite Nanofiber

Two fluid stocks are electrospun in a co-axial manner using a processsimilar to that described in Example 5. The center conduit containszirconium acetate fluid stock of Example 6 and the outer conduitcontains silicon acetate fluid stock of Example 1. The electrospinningprocedure is optionally gas-assisted, e.g., by flowing high velocity gasthrough the outer tube depicted in FIG. 1. The electrospun hybrid fluidstock is calcinated by heating for 2 hours at 600° C. in an atmosphereof air.

Example 9 Ceramic Inclusions

Ceramic inclusion (bentonite) is dispersed with polyacrylonitrile (PAN)in DMF, in a ceramic to polymer ratio of 9:91. Using a gas assistedelectrospinning process, such as described in Example 3, a nanofiber matcomprising nanofibers with a polymer (PAN) matrix and ceramic inclusionsembedded therein is prepared (having a ceramic-to-polymer ratio of9:91). FIG. 3 illustrates SEM (top) and TEM (bottom) images of thenanofibers.

Similar nanofibers are prepared using various amounts of ceramic, e.g.,wherein nanofibers comprise 4.5 wt. % ceramic (e.g., 95.5 wt. % polymer)and 9.5 wt. % ceramic (e.g., 90.5 wt. % polymer).

Example 10 Capacity of Separator Comprising Nanofibers with PAN Matrixand Ceramic Inclusions

Half-cell tests of ceramic-polymer nanofiber separators with a Li-ioncathode (LiCoO₂). Polymer-Ceramic NF Separators 1 and 2 contain 4.5 and9.5 wt. % of bentonite (as described in Example 9). FIG. 4 (panel A)illustrates cycling performance of the capacity. FIG. 4 (panel B)illustrates rate performance over many cycles. Polymer-Ceramic NFSeparators exhibit higher capacity and much better stability over cyclesthan a commercial polyethylene (PE) separator.

Further, half-cell tests of ceramic-polymer nanofiber separators with aSi—C nanofiber Li-ion anode. FIG. 5 illustrates that the cyclingperformance of the capacity of half-cells with the ceramic-polymer NFseparator exhibits higher capacity (10 to 20%) and better stability overcycles than that with a commercial PE separator.

Finally, FIG. 6 illustrates that electrochemical impedance spectroscopy(EIS) tests of ceramic-polymer nanofiber separators demonstrate thatsuch separators exhibit much lower charge transport resistance and muchhigher Li+ diffusion rate than commercial PE separators.

Example 11 Polysilazane

Perhydropolysilazane, polyethyelene oxide (PEO)—MW=100,000, and dibutylether are combined in a PEO:PHPS ratio of 2:1. Using a gas assistedelectrospinning process, such as described in Example 3, a nanofibermat, which is cured at room temperature to provide hybrid nanofiberscomprising an integrated matrix of PEO and silica. FIG. 7 illustratesSEM images of the silica/PEO nanofibers.

Similar nanofibers were also prepared using higher molecular weight PEO(MW=600,000). FIG. 8 illustrates SEM images of such silica/PEOnanofibers. Similarly, other polymers were used in similar processes,such as m-aramid. FIG. 9 illustrates SEM images of such silica/m-aramidnanofibers.

What is claimed is:
 1. A battery separator comprising a nanofiber mat,the nanofiber mat comprising at least one nanofiber, the at least onenanofiber comprising at least one ceramic material.
 2. The batteryseparator of claim 1, wherein the at least one nanofiber comprises acontinuous matrix of ceramic material.
 3. The battery separator of claim1, wherein the separator comprises at least one ceramic material and atleast one polymer material.
 4. The battery separator of any claim 1,wherein the nanofiber is a nanocomposite nanofiber, comprising at leasttwo materials.
 5. The battery separator of claim 4, wherein thenanocomposite nanofiber comprises at least two ceramic materials.
 6. Thebattery separator of claim 5, wherein the at least two ceramic materialsare coaxially layered.
 7. The battery separator of claim 4, wherein thenanocomposite nanofiber comprises at least one ceramic material and atleast one polymer material.
 8. The battery separator of claim 7, whereinthe ceramic material(s) and polymer material(s) are coaxially layered.9. The battery separator of claim 7, wherein the ceramic material(s) andpolymer material(s) form an integrated nanofiber matrix.
 10. The batteryseparator of claim 7, wherein the at least one ceramic material forms acontinuous nanofiber matrix.
 11. The battery separator of claim 4,wherein the nanocomposite nanofiber comprises a first ceramic materialas a core material and a second ceramic material as a sheath material,the sheath material at least partially surrounding the core material.12. The battery separator of claim 5, wherein the at least two ceramicmaterials are independently selected from the group consisting ofsilica, alumina, zirconia, beryllia, ceria, titania, barium titanate,and strontium titanate.
 13. The battery separator of claim 3, whereinthe polymer material is PAN, polyalkeneoxide (e.g., polyethylene oxideor polypropylene oxide), polyalkylene (e.g., polyethylene orpolypropylene), PVA, or a polyacrylate (e.g., polyacrylic acid or PMMA).14. The battery separator of claim 1, wherein the at least one nanofiberis mesoporous.
 15. The battery separator of claim 1, wherein the atleast one nanofiber comprises a mesoporous ceramic matrix.
 16. Thebattery separator of claim 1, wherein the porosity of the nanofiber matis at least 10%.
 17. The battery separator of claim 1, wherein thenanofiber mat comprises at least 50% by weight of ceramic material. 18.The battery separator of claim 1, wherein the at least one nanofiber hasan average diameter of less than 1 micron (e.g., 100 nm to 1 micron, or200 nm to 1 micron).
 19. The battery separator of claim 1, wherein theat least one nanofiber has an average aspect ratio of at least 100(e.g., at least 10,000).
 20. A battery comprising the battery separatorof claim 1, wherein the battery is a lithium ion battery.
 21. (canceled)